1. Field of the Invention
The present invention relates to an electrophotographic image forming device, such as a laser printer, digital photocopier and facsimile machine. More particularly, the present invention relates to a driving apparatus to transmit a driving force from a driving gear to a driven gear, a process cartridge and an image forming device which have the driving apparatus.
2. Description of the Related Art
Generally, electrophotographic image forming devices, such as laser printers, digital photocopiers, and facsimile machines, include one or a plurality of process cartridges. The process cartridge integrates a photoconductive medium unit and a developing unit as a single modular unit. The photoconductive unit has a photoconductive medium that is scanned with laser beams to form an electrostatic latent image thereon, and the developing unit has a developing roller to develop an electrostatic latent image into a developer image. The process cartridge may include only the developing unit as a single modular unit.
The process cartridge is mountable and dismountable for conveniently repairing or replacing the process cartridge.
Also, when the process cartridge includes the photoconductive medium unit and the developing unit as a single modular unit, the developing unit is driven by a driving force transmitted from a driving motor of a body to the photoconductive medium unit, along with the photoconductive medium unit, or it is driven by a driving force transmitted from the driving motor of the body, independently from the photoconductive medium unit.
When the developing unit is driven by a driving force transmitted to the photoconductive medium unit along with the photoconductive medium unit, the developing unit is still driven along with the photoconductive medium unit even when the photoconductive medium idly rotates, that is, when the developing unit does not need to develop an electrostatic latent image formed on the photoconductive medium. The unnecessary driving operation of the developing unit as described above causes friction between a developing roller and a supplying roller for supplying the developing roller with a developer and between the developing roller and a developer regulation blade for regulating the thickness of a developer layer, thereby increasing a developer stress. The increased developer stress deteriorates the uniformity of development and thus degrades quality of image.
Also, because the developing roller and the photoconductive medium are connected to each other through a plurality of gears, the number of teeth of each gear and distances between axes of gears must be taken into account to adjust a velocity ratio between the photoconductive medium and the developing roller to control the developing capability. Therefore, it is difficult to adjust the velocity ratio.
For the above-described reason, a process cartridge that separately drives the developing unit and the photoconductive medium unit is coming into increasing use.
FIG. 1 is an exploded, perspective view illustrating a general process cartridge 1 that separately drives a developing unit and a photoconductive medium unit.
The process cartridge 1 includes a photoconductive medium unit 2, a developing unit 8, and a housing 10.
The photoconductive medium unit 10 includes a drum-type photoconductive medium 4 that is rotatably disposed in a photoconductive medium casing 11 of the housing 10.
As shown in FIG. 3, a first driven gear 6 is disposed on a first end portion 5a of a photoconductive medium shaft 5 that outwardly protrudes from a first lateral frame 11a of the photoconductive medium casing 11 and is covered by a first lateral cover 33. The first driven gear 6 is engaged with a first driving gear 16 of a gear train connected to a driving motor (not shown) disposed in a body when the process cartridge 1 is pushed in the direction indicated by arrow ‘A’ and mounted in the body.
Accordingly, when the driving motor of the body is driven after the process cartridge 1 is mounted in the body as shown in FIG. 2, the first driven gear 6 is rotated by the first driving gear 16 in one direction, for example, in a clockwise direction (see FIG. 3). When the first driven gear 6 is rotated in a clockwise direction, the photoconductive medium 4 coaxially disposed with the first driven gear 6 is rotated in a clockwise direction.
As shown in FIG. 3, a charging roller (not shown) for charging the surface of the photoconductive medium 4, a cleaning member (not shown), such as a cleaning roller or cleaning blade, for cleaning the photoconductive medium 4, and a developing roller 7 of the developing unit 8 are arranged along the photoconductive medium 4.
The developing unit 8 includes the developing roller 7 disposed in a developing casing 13 of the housing 10 and contacting the photoconductive medium 4 with a constant gap therebetween, a supplying roller 14 for supplying the developing roller 7 with a developer, and a developer regulation blade (not shown) for regulating the thickness of a developer layer in contact with the developing roller 7.
A developing roller gear 7b is disposed on a developing roller shaft 7a protruding from a first lateral frame 13a of the developing casing 13 and covered by a second lateral cover 28. The developing roller gear 7b is connected to a second driven gear 19 via a driving force transmission gear 15 and a deceleration gear 17 that are rotatably disposed between the first lateral frame 13a and the second lateral cover 28. A supplying roller gear 14b, which is coaxially disposed with the supplying roller 14, is connected to a lower portion of the driving force transmission gear 15 connected to the developing roller gear 7b. 
As shown in FIG. 3, the second driven gear 19 is engaged with a second driving gear 18 of a gear train connected to the driving motor disposed in the body when the process cartridge 1 is mounted in the body. Accordingly, when the driving motor of the body is driven after the process cartridge 1 is mounted in the body, the second driven gear 19 is rotated in relation to the second driving gear 18 in one direction, for example, in a counterclockwise direction. The clockwise rotational force of the second driven gear 19 is transmitted to the developing roller gear 7b and the supplying roller gear 14b via the deceleration gear 17 and the driving force transmission gear 15. As a result, the developing roller 7 and the supply roller 14 are respectively rotated in a counter clockwise direction.
The housing 10 includes the photoconductive medium casing 11 and the developing casing 13.
The photoconductive medium casing 11 includes the first lateral frame 11a and a second lateral frame 11b, which support shafts of the components of the photoconductive medium unit 2, for example, shafts of the photoconductive medium 4 and the charging roller, and the first lateral cover 33 for sealing the first driven gear 6 disposed on an outer surface of the first lateral frame 11a. 
The developing casing 13 includes the first lateral frame 13a and a second lateral frame 13b, which support shafts of the components of the developing unit 8, for example, shafts of the developing roller 7, the supplying roller 14, the driving force transmission gear 15, the deceleration gear 17, and the second driven gear 19, and the second lateral cover 28 for sealing the developing roller gear 7b, the supplying roller gear 14b, the driving force transmission gear 15, the deceleration gear 17, and the second driven gear 19, which are disposed on an outer surface of the first lateral frame 13a. 
The photoconductive casing 11 and the developing casing 13 are connected to each other via a slide groove 12a formed in a lower portion 12 (see FIG. 3) of the first lateral frame 11a of the photoconductive medium casing 11 to receive the developing roller shaft 7a, and a fixing hole (not shown) of a fixing part (not shown) formed at a rear portion of the second lateral frame 13b of the developing casing 13 to receive and rotatably support a second end portion (not shown) of the photoconductive medium shaft 5.
First and second protruding seating members 20 and 21 and first and second locking springs 23 and 24 are disposed at portions of the body that correspond to the first and the second end portions 5a of the photoconductive medium shaft 5 that protrude outwardly from the rear portions of the first and the second lateral frames 11a and 11b of the photoconductive medium casing 11.
First and second mounting guide recesses 29 (only the first mounting guide recess 29 is illustrated in the drawings) are formed in front portions of the second lateral cover 28 of the developing casing 13 (the right in the drawings) to receive and guide first and second mounting protrusions 26 and 27 formed on the body, when the process cartridge 1 is mounted in the body.
Accordingly, when the process cartridge 1 is mounted in the body by being pushed in the direction indicated by arrow ‘A’ as shown in FIG. 1, the first and second mounting guide recesses 29 of the process cartridge 1 receive and guide the first and second mounting protrusions 26 and 27. Also, the first and second end portions 5a of the photoconductive medium shaft 5 upwardly push supporting ends of the first and second locking springs 23 and 24, which are seated in the first and second protruding seating members 20 and 21, and then are elastically locked into the first and second protruding seating members 20 and 21 by the supporting ends of the first and the second locking springs 23 and 24.
On the other hand, when the process cartridge 1 is pulled out in the direction indicated by arrow ‘B’ and is dismounted from the body, the first and second mounting guide recesses 29 guide the first and the second mounting protrusions 26 and 27 to remove them, and the first and the second end portions 5a of the photoconductive medium shaft 5 upwardly push the supporting ends of the first and the second locking springs 23 and 24 against the elastic force of the first and second locking springs 23 and 24, thereby escaping from the first and second protruding seating members 20 and 21.
However, according to the conventional process cartridge 1 as constructed above, because the first driven gear 6 transmits a driving force to only the photoconductive medium 4 and the second driven gear 19 transmits a driving force to both the developing roller 7 and the supplying roller 14 through the plurality of gears 17, 15, 7b, 14b, a load torque of the second driven gear 19 in response to a rotational force of the second driving gear 18 connected to the gear train connected to the driving motor of the body, in other words, a load torque of the second driven gear 19 in response to a driving torque of the second driving gear 18 is greater than a load torque of the first driven gear 6 in response to a driving torque of the first driving gear 16.
Also, the photoconductive medium 4 is supported by the first and second lateral frames 11a and 11b of the photoconductive medium casing 11, whereas the second driven gear 19 is supported by only the first lateral frame 13a of the developing casing 13.
Accordingly, when a driving force of the driving motor of the body is transmitted from the second driving gear 18 to the second driven gear 19, the second driven gear 19 is subjected to a force ‘F1’ that is generated by a load torque in response to the driving torque of the second driving gear 18. Although the process cartridge 1 at both sides is supported at the first and second protruding seating members 20 and 21 and the first and the second mounting protrusions 26 and 27 by the first and the second end portions 5a of the photoconductive medium shaft 5 and the first and second mounting guide recesses 29, the first lateral frame 13a of the developing casing 13 is pushed by the force ‘F1’ generated by the load torque because the first lateral frame 13a of the developing casing 13, to which the second driven gear shaft 19a is fixed, and the first lateral frame 11a of the photoconductive medium casing 11 are movably fixed each other through the slide groove 12a and the developing roller shaft 7a. As a result, the second driven gear 19 does not maintain a constant position and changes a distance between its axis and the axis of the second driving gear 18, depending on the change in the load torque. Thus, the second driven gear shaft 19a changes position. Simultaneously, the process cartridge 1 suffers from a warp deformation, such that the photoconductive medium casing 11 and the developing casing 13 become warped downwardly with respect to the developing roller shaft 7a. 
Also, because the process cartridge 1 is in the shape of both arm beams, such that both ends thereof are supported at the first and the second protruding seating members 20 and 21 and the first and the second mounting protrusions 26 and 27 by the first and second end portions 5a of the photoconductive medium shaft 5 and the first and second mounting guide recesses 29, the second driven gear 19, which is located in the middle of the process cartridge 1, generates a vibration due to repulsive forces F2 and F3 of the first and the second protruding seating members 20 and 21 and the first and the second mounting protrusions 26 and 27 in response to the driving torques of the first and the second driving gears 16 and 18. Accordingly, the second driven gear shaft 19a greatly changes position due to the load torque of the second driven gear 19.
When the second driven gear shaft 19a changes position due to the vibration and thus the process cartridge 1 is deformed, the developing roller 7, which is fixed to the first and second lateral frames 13a and 13b of the developing casing 13 to maintain a constant gap with respect to the photoconductive medium 4 using a gap ring (not shown), changes a gap size with respect to the photoconductive medium 4. As a result, there occurs a defect, such as jittering, in an image obtained by the process cartridge 1 due to the change in the gap between the developing roller 7 and the photoconductive medium 4.
Accordingly, a need exists for an improved driving apparatus for an image forming device.